There’s a decent Wikipedia entry on them: https://en.wikipedia.org/wiki/Skull_and_Bones?wprov=sfti1

Tl;dr is they are a secret society at Yale comprised of legacy kids (I believe both presidents Bush were members, for example) that has initiation rites that are supposed to be humiliating which creates a bond. Apparently they have changed quite a bit from their heyday in the first half of the twentieth century.